It is conventional to set the air velocity for a drying chamber at a certain level within the stack of wood based upon the geometry of the drying chamber, the conveying capacity of the fans, the aerodynamic resistance of the heating radiator, the size and arrangement of the stack of wood, as well as stack configuration factors. During the drying each fan is set or adjusted according to the type of wood, the wood thickness and the wood moisture level, as well as the desired quality to be obtained by the adjustment of the speed of the fan or fans.
With this computation, however, a number of parameters or factors, for instance the precision of the stacking, deviations called air short circuits, the surface area condition of the wood, modifications of the layers of wood as a result of shrinkage, unfit or improper stack strip ledges or borders and the like, are not taken into consideration. Additionally, the flow velocities between the individual cut wood layers can deviate considerably from the desired levels. With greater total stack height, the differences, in the flow velocities between the individual partial flows penetrating between the wood layers, increase a great deal. These differences in the flow velocities cause individual areas of the wood to dry at different speeds.
Depending on whether the measuring points for control of the drying cycle are selected to be in better or in poorer air flowthrough areas, the measurements, and therefore, the conventional processes and apparatus can produce low quality goods as a result of too high or too low levels of wood moisture. Too high moisture levels lead to discolorations and the formation of mold. Too low moisture levels can result in overdried goods, and with extended drying time, can also raise the cost of the process. If the charges of wood being processed have different original moisture levels, additional drying damages can occur, such as planking or revetting, warped areas and crevices, especially if the more moist charges are being air-dried more strongly than the drier, standard charges.